1. Field of the Invention
This invention relates generally to furnaces for heat treating of metal parts or materials and more particularly, it relates to a dual belt furnace which includes a finely woven wire-mesh belt for carrying small metal parts between the entrance end and the exit end of the furnace and a cast link belt of a high load-carrying capacity for supporting the wire-mesh belt within the heating zones of the furnace.
2. Description of the Prior Art
In the heat treatment of high-precision, high finish small metal parts, it is a general requirement that the metal be softened part way through the process so as to enable further processing of the metal parts. Traditionally, it has been the practice heretofore in the heat treatment of various metal parts to use a process referred to as "batch handling." In this batch handling process, the metal parts were packed with the usual carburizing compound in cast-metal pots or drums. The drums were then placed in a furnace and heated to and held at the desired carburizing temperature for the requisite time. After the expiration of the appropriate time interval, the drums were removed from the furnace and dumped and allowed to cool before they were re-packed. This process required the use of a number of laborers, for example, one to re-pack the cold drums, one to remove the drums from the furnace, one to dump the contents of the drum into a sieve for separating any medium from the metal parts, etc. As a result of the foregoing, there have a been a series of developments beginning from the earliest days of this century which are aimed at eliminating the need for this costly batch handling, thereby improving the manner in which the metal parts can be heat treated in a furnace.
A first type of heat-treating systems developed to replace the relatively primitive batch-style furnaces were rotary retort furnaces. The rotary retort furnace includes a cylindrical retort having dimensions of approximately 30 inches in diameter and 15 feet in length. The cylindrical retort is arranged so that the heat can be externally supplied, while the retort itself can be rotated about a longitudinal axis. Within the retort, there is provided a spiral flight or guide-fence, approximately 3 inches in height, which is attached to the wall at one edge. As the retort is rotated, the parts within the retort will be driven along length as by an auger. The parts are thus subsequently heated and as they reach the end of the retort, they are transferred to a liquid or gas quench environment.
While this heat-treating system has the advantage of providing a controlled atmosphere and being somewhat high energy efficient, it has limitations because only relatively small quantities can be conveyed through the retort furnace and throughput can only be increased by increasing the length of the retort. Further, as the parts are continually subjected to a tumbling action in which the parts are thrown against each other and against the spiral flights by operation of the retort, there is the obvious possibility of surface damage to the parts. Moreover, this problem becomes worse when the furnace is increasingly heavily loaded. Another problem associated with the tumbling action is that only units of one particular type of the part can be heat treated at a time, unless a costly sorting operation is performed afterwards.
A second type of heat-treating systems developed to replace the batch handling process were continuous conveyor furnace designs in which the parts are moved steadily through a heating chamber. In such furnace designs, the travel of the conveyor can be made to extend beyond the confines of the heating zone. Such a design permits the easy loading onto the conveyor at the input end of the furnace, and the easy delivery to a quench bath or other receiving station at the output end thereof.
Conveyor furnace technology has developed along three lines since those early years: First, there came the mesh belt conveyor furnace, in which a temperature-resistant metal wire is used to weave or otherwise construct the conveyor belt. This belt followed a more or less convoluted course, serving to pick up parts outside the input end of the furnace, carry them through the furnace for the required time, and deliver them to some receiving station at the output end. The belt also had to pass through the belt drive system, however this might be arranged.
Such a belt would often be carried between two parallel chains or cables; these items served to transmit the driving force as well as to provide mechanical support at the edges of the belt. Such a belt was ideal for small, light-weight parts and provided thermal efficiencies close to those of retort-type furnaces in that the belt itself, which must of necessity be subjected to continual heating up and cooling down as it enters and leaves the furnace, would have only a small thermal capacity. Because of the nature of the belt, there was also good circulation of atmosphere provided through the belt and thus around the parts.
For heavier and denser part loading, the mesh belt technology offered inadequate mechanical strength, particularly when it is considered that the belt must operate at elevated temperatures. In these cases, a second type of conveyor furnaces were designed in which the mesh of the belt was superseded by a relatively massive construction of cast links, interlocking with each other. Such a form of construction, while providing all the mechanical strength that could be desired, and while allowing circulation of the furnace atmosphere through the interstices between the links, suffered from an accompanying disadvantage in that the cast links had a relatively high thermal disadvantage in that the cast links had a relatively high thermal capacity. As the belt had to continually enter and leave the furnace in order to pick up and deliver parts, the result was a furnace of lower energy efficiency. This design was also ill-suited to small, high-density parts as these can fall into the interstices between the links, becoming lost or damaged in the process, and sometimes damaging the belt.
A third form of furnace construction, developed within the last decade, relied upon a series of plates, pans or buckets replacing the belts in the above-described designs. These buckets offered the particular facility of being able to dump their contents into any one of several receiving stations, or to continue to carry their contents to another furnace section if desired. The design thus had great flexibility, although the heat efficiency was generally poorer than for the mesh belt design. Further, insofar as parts would be heaped in the pans or buckets, there was an opportunity for considerable surface damage to occur, which was generally not the case in belt furnaces, where parts would usually be loaded in thin layers. Finally, this design was less conducive to good circulation of the furnace atmosphere around the parts.
It will be noted that none of these technologies is ideal for a production heat-treated situation requiring a high throughput of high-density parts whose surface finish is of the utmost importance to the success of the operation. The rotary retort has acceptable thermal efficiency but damages the parts at high throughputs; the mesh-belt has inadequate mechanical strength; the link-belt system has poor thermal efficiency and is ill-suited to the handling of small parts; and the bucket-conveyor design has a thermal efficiency almost as poor, and threatens to damage the parts as well, while restricting atmospheric circulation.
As a result, plants devoted to the production of small high-precision parts requiring a high surface finish without mechanical imperfections cannot find the ideal furnace for the heat treating of their product. As can be seen by the foregoing, every technology so far available presents one or more disadvantages from the viewpoint of heat treating such parts.
A state-of-the-art search directed to the subject matter of this application uncovered the following U.S. Pat. No.:
______________________________________ 932,945 2,007,862 1,792,456 3,565,409 1,922,908 4,402,494 ______________________________________
There is disclosed in U.S. Pat. No. 1,792,456 to Charles T. Willard and Richard Kaier issued on Feb. 10, 1931, a metal-treating furnace which includes a tubular retort disposed within a suitable heating chamber and an endless flexible wire-fabric belt-conveyor. The belt-conveyor runs over a loading platform and through the retort in contact with the continuous bottom wall of the platform and retort.
There is disclosed in U.S. Pat. No. 1,922,908 to Spencer A. Coleman issued on Aug. 15, 1933, an apron conveyor which is formed of sections of foraminous material such as a wire mesh or perforated sheet metal that may be conveniently assembled. The end portions of the foraminous sections are offset and project outwardly from the backside of the conveyor. Hinged members are secured to the offset portions. Stiffening plates are secured to the offset portions and the hinged members so as to provide a substantially continuous supporting surface.
In U.S. Pat. No. 2,007,862 to Alpheus O. Hurxtal issued on Jul. 9, 1935, there is taught a mesh screen conveyor which is formed of a series of screen sections. Each of the sections is comprised of relatively superimposed layers of fine mesh screening and a more coarse open mesh. The coarser mesh is used to provide support for the fine mesh screening when the conveyor is carrying a heavy load.
In U.S. Pat. No. 3,565,409 to Jacob H. Beck issued on Feb. 23, 1971, there is taught a conveyor system which is arranged to transport materials being treated through several zones of the furnace muffle. The conveyor includes a plurality of hinged trays attached to a movable link chain. The trays are arranged to either drop their contents into a quench bath or to convey their contents to an air-rich atmosphere depending upon the presence or absence of a removable bridge section. The chain passing through the furnace muffle is supported in a horizontal material-retaining position by the floor of the muffle. The link chain includes spaced-apart link members, each pair of link members being pivotally connected to a like pair of adjacent members by pivot rods. The tray is pivotally mounted on the pivot rods for retaining the materials to be treated.
The remaining patents listed above but not specifically discussed are believed to be of only general interest and show the state of the art in furnaces for heat treatment of materials.
However, none of the prior art uncovered in the search disclosed a dual belt furnace like that of the present invention which includes a wire-mesh belt for carrying small metal parts and a cast link belt of a high load carrying capacity for supporting a mesh belt within the heating zones of the furnace.